This is probably a big misunderstanding - and mistake - on my part.

When first playing 2.0, I was a SMN/SCH (solo/party), and during the SMN intro quest I inferred this...

Quote Originally Posted by Y'mhitra
According to the records we recovered, the summoners of old would perform these rites in a land in which the naturally dominant element matched the aspect of the avatar they wished to call forth. Only in such a place might one sufficiently shift one's aetheric balance in the desired direction.
The element of fire is especially strong here in the Sagolii Desert─I can think of no land more fitting in which to summon an incarnation of the Lord of the Inferno.
... to mean that, due to the proximity to the Amalj'aa dwelling (Zahar'ak), the Sagolii Desert itself was a product of repeated Ifrit summons through the ages. Combined with the knowledge that prior to Silvertear the beast tribes traded their crystals with the Eorzean city-states instead of hoarding them for summoning rituals, that led me to the conclusion that primals were possible to summon beforehand... it just wasn't common, due to both Midgardsormr's seal and the crystal stocks being kept low through trade. The truth is that the latter was seemingly absolute, though, and the latter was unimportant to the state of things.

(60% of the time, I'm right every time.)

Anyway, enough of people overindulging my ignorance. With that information, inferences can be made!

1. The Garleans are not known to have encountered a primal during their conquest prior to Eorzea. Solus saw the Burn, concluded that it must have been the result of eikons (presumably based on information gleaned from Allag), and declared their already-underway conquest of the world justified to stop primals.
2. Primals were not known to have been summoned during the Sixth Astral Era prior to the Battle of Silvertear Skies, aside from a possible instance during the consolidation of Imperial power in Gyr Abania.

This means that the Burn is not the result of primal activity and/or that it is older than the Sixth Astral Era.

If nobody saw the Burn come into existence, that means nobody can say for certain that it's the result of primal activity. If it wasn't possible to summon primals during the Sixth Astral Era prior to Silvertear, the Burn would have to predate that if it was created by primals as it was discovered by the Garleans before that event.

... since, for some reason, it was possible to summon in previous Astral Eras (the Third, at least, since that's when the Warring Triad, Odin, and Bahamut were summoned). For every answer, three questions.